Current methods for membrane electrode assembly (MEA) fabrication are decal transfer (described, e.g., in U.S. Pat. No. 5,211,984, issued to Wilson et al.) and gas diffusion electrode transfer methods (described, e.g., in U.S. Pat. No. 5,998,057, issued to Koschany et al.). Both methods involve a hot pressing procedure at a relatively high temperature (125-300° C.). The high temperature process has been deemed necessary to form a durable interface between the membrane and electrode, one reason for which is that the temperature must be higher than 120° C., which is the softening temperature of the current standard industrial fuel cell membrane material comprising perfluorinated sulfonic acids (PFSAs) (e.g. Nafion™).
Another desirable membrane for MEAs is hydrocarbon-based polymers. For this type of membrane, PFSA ionomer is the preferred material to bind the membrane to the electrode, which helps offset the low permeability of the hydrocarbon membrane. However, a significant concern for using hydrocarbon based membrane-PFSA bonded electrode assemblies is that the two materials are not fully chemically compatible. In addition, most thermally stable hydrocarbon based membranes have a softening temperature higher than 250° C. The chemical incompatibility between hydrocarbon membrane and PFSA ionomer and higher softening temperature make it difficult to form a durable interface between the two components, which often results in delamination under operating conditions and subsequent performance degradation. This issue is described in a number of publications, including Chen et al., “Fuel cell performance of polyetheretherketone-based polymer electrolyte membranes prepared by a two-step grafting method,” J. Membrane Science, v. 319, issue 1, pp. 1-4 (2008); Gubler et al., “Performance and Durability of Membrane Electrode Assemblies Based on Radiation-Grafted FEP-g-Polystyrene Membranes,” Fuel Cells, v. 4, issue 3, pp. 196-207 (2004), and Jörissen et al., “New membranes for direct methanol fuel cells,” J. Power Sources, v. 105, issue 2, pp. 267-273 (2002). Therefore, few reports have published fuel cell performance of MEAs using hydrocarbon membranes, and the performance results that have been published show neither superior performance to MEAs having a Nafion™ membrane, nor an increase in stability.
Yet another concern associated with high temperature processing is degradation of polymer electrolytes and/or changing electrode structure in both MEAs using PFSA and hydrocarbon membranes. To prevent polymer degradation during hot temperature pressing, the proton form of the membrane and ionomer may be converted to the more thermally stable salt form (e.g. sodium or tetra butyl ammonium (TBA)) before hot pressing. The conversion process adds to the complexity of MEA fabrication in terms of, for example, material cost, processing time and/or energy consumption. The electrode structure can be also adversely influenced by high temperature processing, resulting in lower fuel cell performance and longer catalyst activation time for thermally aged MEAs.
There exists a need, therefore, for methods of making stable MEAs at lower temperatures which have high-efficiency, high-durability, and good compatibility between the membrane and the material used to bind the electrodes to the membrane, which results in good mechanical stability. There exists a further need for a method of making good quality MEAs which utilizes a hydrocarbon-based membrane.